Angry Birds 2 downloaded 1M 12 hours after launch

Angry Birds 2, the first official sequel to one of the best-selling mobile apps, was released worldwide to great fanfare and interest Thursday, but new features including in-app purchases have alienated some fans.

The game's Finnish developers, Rovio Entertainment, claimed on Twitter that Angry Birds 2 has reached one million downloads just 12 hours after launch.

The original game was released in 2009. Since then, Rovio Entertainment has released 12 games in the Angry Birds series, as well as three spin-off games, and has branched out into merchandise, cartoons and an upcoming film scheduled for 2016. The developers claim the game series has a collective three billion downloads.

Angry Birds sequel takes flight

The new game, which is free to download, features updated graphics and new gameplay mechanics, but has also introduced in-app purchases and a "life" system: players have five "lives" but lose one each time they fail a level and cannot play if they reach zero.

Players can either wait 30 minutes for their "lives" to regenerate, or they can buy more using gems earned during gameplay or bought with real-world cash.

Creative director Patrick Liu defended the inclusion of in-app purchases during an interview with the BBC.

"We obviously put a lot of time and effort into making it feel fair, that you feel justified in spending money, that you get your value from that spend," he said. "We have no interest in tricking or exploiting our fans."

Source: Rovio Mobile

But reviews left by users on the game's iTunes and Google Play stores, as well as comments on Twitter, show fans are disappointed with the changes.

Rovio Entertainment faces other challenges in trying to make money from their game.

"Despite the game's huge popularity, Rovio has struggled to keep pace with the monetization success of mobile games competitors such as King, Supercell and others," explained Jack Kent, director of mobile media for IHS Technology. "Even when Rovio did adopt freemium features and in-app purchases it was not able to hit the top of the revenue charts because the features it offered were more limited and did not encourage either the repeat spending or high value purchases that are necessary for mobile app success.

"To reach the top tier of mobile games revenues it will need to focus more on driving in-app revenues, but to do so it must make sure they fit the nature of its gameplay and be wary not alienate the core audience."

Follow us on Twitter: @CNBCWorld